I general I agree with Tim, but provided it doesn't kill your protein,
oxidation of selenomethionine can actually enhance your anomalous signal
considerably (http://dx.doi.org/10.1107/S0907444901008666).
The reason for this is because the "white line" peak in the SeMet x-ray
absorption spectrum is actually a pre-edge feature. Although the "edge"
itself represents the point when the incoming photon has just enough
energy to completely eject a core electron from the Se atom, it is also
possible for the electron to be not exactly "ejected" but merely
promoted to an empty orbital with energy a few eV below that of a free
electron in a vacuum. It is because of these extra landing sites that
the near-edge structure of the x-ray absorption specturm (XANES or
NEXAFS) can be influenced by chemistry. Specifically, the "peak" of the
normal SeMet spectrum is a 1s-4p transition
(http://dx.doi.org/10.1021/es00009a043
http://dx.doi.org/10.1107/S0909049506048898), and the more oxidized the
Se atom is, the less occupied the 4p level becomes and the bigger the
while line gets. And, of course, the bigger the while line the more f"
phasing signal you get.
Unfortunately, the extra oxygens stuck on the Se can sterically disrupt
the local environment of the SeMet, and fear of this is probably why so
many people have not tried it. However, if you're desperate for that
extra little "boost" of phasing signal, it might be a refreshing change
to do the opposite of trying to keep your protein from oxidizing all the
time. Perhaps even adding a dash of H2O2. The worst your crystals can
do is not diffract.
-James Holton
MAD Scientist
On 7/1/2014 8:54 AM, Tim Gruene wrote:
Dear Maher,
as far as I understand, the anomalous scattering comes from inner shell
electrons, not the valence electrons. So while you might notice a slight shift
in the peak wavelength, the strength of the signal will only reduce if you
crystal order suffered over time, but not from any oxidation of the Se.
Best,
Tim
On Tue, Jul 01, 2014 at 10:51:20AM -0500, Maher Alayyoubi wrote:
Hi everyone,
Would anyone know for how long Selenomethionine derivative crystals are
good if kept in plate at RT. In other words, would SE loose its scattering
properties due to oxidation over time? I have SElmet crystals that have
been lying in a plate for 2 months by now so I was wondering if they are
still worthwhile the effort of collecting new data from them.
Thank you very much,
Maher
